Alzheimer's disease is a degenerative disorder of the human brain. Clinically, it appears as a progressive dementia. Its histopathology is characterized by degeneration of neurons, gliosis, and the abnormal deposition of proteins in the brain. Proteinaceous deposits (called "amyloid") appear as neurofibrillary tangles, amyloid plaque cores, and amyloid of the congophilic angiopathy. [For reviews, see, Alzheimer's Disease, (B. Reisberg, ed., The Free Press 1983).]
While there is no general agreement as to the chemical nature of neurofibrillary tangles, the major constituent of both the amyloid plaque cores and the amyloid of the congophilic angiopathy has been shown to be a 4500 Dalton protein originally termed .beta.-protein or amyloid A4. Throughout this document this protein is referred to as .beta.-amyloid peptide or protein.
.beta.-amyloid peptide is proteolytically derived from a transmembrane protein, the amyloid precursor protein. Different splice forms of the amyloid precursor protein are encoded by a widely expressed gene. see, e.g., K. Beyreuther and B. Muller-Hill, Annual Reviews in Biochemistry, 58:287-307 (1989). .beta.-amyloid peptide consists, in its longest forms, of 42 or 43 amino acid residues. J. Kang, et al., Nature (London), 325:733-736 (1987). These peptides, however, vary as to their amino-termini. C. Hilbich, et al., Journal of Molecular Biology, 218:149-163 (1991).
Because senile plaques are invariably surrounded by dystrophic neurites, it was proposed early that .beta.-amyloid peptide is involved in the loss of neuronal cells that occurs in Alzheimer's disease. B. Yankner and co-workers were the first to demonstrate that synthetic .beta.-amyloid peptide could be neurotoxic in vitro and in vivo. B. A. Yankner, et al., Science, 245:417 (1989); See, also, N. W. Kowall, et al., Proceedings of the National Academy of Sciences, U.S.A., 88:7247 (1991). Other research groups, however, were unable to consistently demonstrate direct toxicity with .beta.-amyloid peptide. See. e.g., Neurobiology of Aging, 13:535 (K. Kosik and P. Coleman, eds. 1992). Even groups receiving .beta.-amyloid peptide from a common source demonstrate conflicting results. D. Price, et al., Neurobiology of Aging, 13:623-625 (1991)(and the references cited therein).
As mentioned supra, cells have alternative mechanisms for processing amyloid precursor protein which can result in the formation of the .beta.-amyloid protein and subsequently, the senile plaques. It is likely that this alternative processing route occurs in the lysosomes. It has been found that compounds that inhibit lysosomal enzymes inhibit the fragment formation. See, e.g., Science, 255:689 (1992).
A lysosome is a membranous reservoir of hydrolytic enzymes responsible for the intracellular digestion of macromolecules. Lysosomes are known to contain approximately forty hydrolytic enzymes, including proteases, nucleases, glycosidases, lipases, phospholipases, phosphatases and sulfatases. These enzymes are all acid hydrolases which are optimally active at about pH 5. Therefore, it is necessary to determine which enzyme or enzymes are responsible for this alternative processing of the amyloid precursor protein and the consequent formation of the .beta.-amyloid protein.
Abnormally high concentrations of the proteases cathepsins D and B have been observed in the brains of patients with early-onset Alzheimer's disease. Yu Nakamura, et al, Neuroscience Letters, 130, 195-198 (1991). The cathepsins are a family of hydrolase enzymes that are usually located in the lysosomes. These enzymes are endopeptidases with an acidic optimum pH. Cathepsin A is a serine carboxypeptidase, cathepsin C [EC 3.4.14.1] is a dipeptidyl peptidase, cathepsin D [EC 3.4.23.5] is an aspartyl protease, and cathepsin B.sub.2 [EC 3.4.16.1] is a serine carboxypeptidase. Cathepsin B [EC 3.4.22.1] (also known as cathepsin B.sub.1) and cathepsin L [EC 3.4.22.15] are thiol proteases having activity within the lysosomes.
Abnormally high concentrations of the proteases cathepsins D and B have been observed in the brains of patients with early-onset Alzheimer's disease. Yu Nakamura, et al., Neuroscience Letters, 130:195-198 (1991). Furthermore, elevated activity for cathepsin D has been observed in the brains of Alzheimer's patients. M. Takeda, et al., Neurochemistry Research. (abstract), 11:117 (1986). Cathepsin D is a lysosomal endoprotease that is present in all mammalian cells. See, e.g., "Proteinases in Mammalian Cells and Tissues," ed. (A. J. Barret, ed. 1977) pp. 209-248. It is the only aspartyl protease that is known to be a lysosomal enzyme.
It has been found that inhibition of cathepsin D using an aspartyl protease inhibitor reduces the formation of .beta.-amyloid protein and the resultant senile plaque. These and other aspects of the present invention are discussed in greater detail below.
Because of the debilitating effects of Alzheimer's disease there continues to exist a need for effective treatments. This invention provides methods for the treatment of Alzheimer's disease in mammals. Specifically, this invention provides methods of using inhibitors which are specific for the subgroup of cathepsins which are aspartyl proteases as a treatment for Alzheimer's disease.